Previous work on this project has shown that the major outer membrane protein (MOMP) of Chlamydia trachomatis organisms is the serotyping antigen of this important group of human pathogens. It was also shown that polyclonal IgG antibody specific for the MOMP neutralized chlamydial infectivity in vitro and that this neutralization was primarily serotype specific. Currently this project is focused on the identification and molecular characterization of these potentially important surface antigens of C. trachomatis organisms. The approach has been to raise monoclonal antibodies against serotype and serogroup epitopes present on the MOMP. These antibodies were radioiodinated and used as probes to map the native antigenic topography of MOMP on viable chlamydiae. The data show that only monoclonal antibody directed against the serotype-specific epitope of the MOMP is capable of binding to the surface of viable organisms. These same antibodies were also found to reduce in vitro infectivity of the parasite. In contrast, radioiodinated monoclonal antibodies against a species-specific epitope of the MOMP or a genus-specific epitope located on chlamydial lipopolysaccharide did not bind viable chlamydiae or reduce infectivity. These data suggest that the surface-exposed antigenic domains of the MOMP that confer serotype specificity are promising candidate antigens for a C. trachomatis subunit or peptide vaccine. A 9K dalton cyanogen bromide fragment of MOMP that binds to the MOMP serotype-specific monoclonal antibody has been identified by immunoblotting analysis (see project #Z01 AI 00233-02 LMSF). This peptide fragment is being isolated by immunosorption using solid phase bound type-specific monoclonal IgG antibody. Once isolated, the amino acid sequence of the peptide or its succinylated-trypsin digested fragments will be determined. It is then proposed to synthesize the immunological active peptide and to initiate appropriate animal model studies to evaluate its vaccine potential.